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Technical question about a zoom lens
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<blockquote data-quote="WayneF" data-source="post: 392075" data-attributes="member: 12496"><p>The marked focal length is at infinity focus. The focal length increases as the lens is extended to focus closer (lens is farther from sensor plane). This is not much issue at mild close ups (up to say 1:10, but it is at f/1.1). Generally at 1:1, the focal length becomes 2x the focal length at infinity. Also note that 1:1 means subject distance is equal to sensor distance (focal length). Since f/stop number = focal length / aperture diameter, then 2x focal length means 2x focal length number, which is as if two f/stops stopped down (at same maximum diameter, so f/2.8 1:1 becomes f.5.6... but internal focus lenses might be slightly less, maybe f/5). Macro lenses today know to keep up with the correct f/number, and they report the actual adjusted number, so at closer distances, it appears not to open to f/2.8. It is fully open, it is just not f/2.8 at extreme closeup.</p><p></p><p>Of course, we would not use f/2.8 at 1;1, but the f/16 we do use computes f/8 at infinity. f/32 is the previous f/16.</p><p></p><p>Near page bottom at <a href="http://www.scantips.com/lights/fieldofview.html" target="_blank">Field of View (FOV) Calculator</a> is the "thin lens equation" which is a simple explanation of this 2x fact.</p></blockquote><p></p>
[QUOTE="WayneF, post: 392075, member: 12496"] The marked focal length is at infinity focus. The focal length increases as the lens is extended to focus closer (lens is farther from sensor plane). This is not much issue at mild close ups (up to say 1:10, but it is at f/1.1). Generally at 1:1, the focal length becomes 2x the focal length at infinity. Also note that 1:1 means subject distance is equal to sensor distance (focal length). Since f/stop number = focal length / aperture diameter, then 2x focal length means 2x focal length number, which is as if two f/stops stopped down (at same maximum diameter, so f/2.8 1:1 becomes f.5.6... but internal focus lenses might be slightly less, maybe f/5). Macro lenses today know to keep up with the correct f/number, and they report the actual adjusted number, so at closer distances, it appears not to open to f/2.8. It is fully open, it is just not f/2.8 at extreme closeup. Of course, we would not use f/2.8 at 1;1, but the f/16 we do use computes f/8 at infinity. f/32 is the previous f/16. Near page bottom at [URL="http://www.scantips.com/lights/fieldofview.html"]Field of View (FOV) Calculator[/URL] is the "thin lens equation" which is a simple explanation of this 2x fact. [/QUOTE]
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